Quinacridone, also referred to as 5,12-dihydroquino[2,3-b]acridine-7,14-dione, is a well-known pigment having the formula I ##STR1##
Quinacridone is known to exist in three major polymorphic modifications. The alpha form, which is disclosed in U.S. Pat. No. 2,844,484, and the gamma forms which are disclosed in the U.S. Pat. Nos. 2,844,581; 2,969,366; 3,074,950 and 5,233,624 respectively, are red, while the beta form is a violet pigment.
Several patents describe various forms of beta quinacridone pigments. For example U.S. Pat. Nos. 4,857,646; 5,229,515 and 5,281,269 describe a beta-I form which is distinguished from the previously known beta-II form described in U.S. Pat. No. 2,844,485 by its X-ray-diffraction pattern and color characteristics. The beta-I form is characterized by its opaque magenta color versus the transparent violet quinacridone pigments of the beta-II form. Both forms are commercially available.
The present invention relates to a novel beta quinacridone pigment having unique and surprising color characteristics. In particular, the inventive pigment is characterized by its masstone and tint reflection spectra from 400 to 700 nm, its x-ray-diffraction pattern, which is a beta form; its specific surface area, which is typically in the range of 17.+-.4 m.sup.2 /g, and its primary pigment particle size, which is generally in the range of 0.1 to 3 .mu.m.
The inventive pigment shows, depending on its application media, its pigmenting concentration and other co-applied pigments, a unique color. Particularly interesting color shades are generated by applying the pigment together with effect pigments such as pearlescent mica or metallic pigments. Such shades show a strong unique color flop from a violet to a red. This is very surprising since the masstone of the pigment itself has a brownish-bronze appearance. Due to the outstanding fastness properties, its excellent Theological properties and unique color characteristics, it is highly suited for use in plastics and coatings applications, particularly in automotive coating systems.